Observation of the P11 state of charmonium

The spin-singlet P-wave state of charmonium, h(c)(P-1(1)), has been observed in the decay psi(2S)->pi(0)h(c) followed by h(c)->gamma eta(c). Inclusive and exclusive analyses of the M(h(c)) spectrum have been performed. Two complementary inclusive analyses select either a range of energies for the photon emitted in h(c)->gamma eta(c) or a range of values of M(eta(c)). These analyses, consistent with one another within statistics, yield M(h(c))=[3524.9 +/- 0.7 (stat)+/- 0.4 (sys)] MeV/c(2) and a product of the branching ratios B-psi(psi(2S)->pi(0)h(c))xB(h)(h(c)->gamma eta(c))=[3.5 +/- 1.0 (stat)+/- 0.7 (sys)]x10(-4). When the eta(c) is reconstructed in seven exclusive decay modes, 17.5 +/- 4.5 h(c) events are seen with an average mass M(h(c))=[3523.6 +/- 0.9 (stat)+/- 0.5 (sys)] MeV/c(2), and B psi Bh=[5.3 +/- 1.5 (stat)+/- 1.0 (sys)]x10(-4). If combined, the inclusive and exclusive data samples yield an overall mass M(h(c))=[3524.4 +/- 0.6 (stat)+/- 0.4 (sys)] MeV/c(2) and product of branching ratios B psi Bh=[4.0 +/- 0.8 (stat)+/- 0.7 (sys)]x10(-4). The h(c) mass implies a P-wave hyperfine splitting Delta M-HF(1P)equivalent to < M(1(3)P)>-M(1(1)P(1))=[1.0 +/- 0.6 (stat)+/- 0.4 (sys)] MeV/c(2).